Lebanon's President Michel Aoun has said nothing justifies the apparent detention of Saad Hariri in Saudi Arabia.

In a statement on Wedneday, Aoun called the incident a Saudi act of aggression.

Hariri, a Sunni Muslim politician and longtime ally of Saudi Arabia, suddenly announced his resignation as Lebanon's prime minister during a visit to Riyadh last week.

He has been promising to return home soon but Saudi leaders say they fear for his safety if he does.

In an interview on Sunday night with the Future TV station owned by him, Hariri drew attention to the risk of economic sanctions and a threat to the livelihoods of Lebanese expatriate workers in Saudi Arabia and its Gulf allies.

Hariri's latest pledge to return came on Tuesday via Twitter after he met the visiting head of Lebanon's Maronite Christian Church in Riyadh, Patriarch Beshara al-Rai.

Rai became the first Lebanese public figure to visit Saudi Arabia since Hariri announced his resignation in a televised speech from Riyadh on November 4.

In that speech, Hariri blamed interference in Lebanon by Iran and its Lebanese ally Hezbollah for his decision, adding that he feared an assassination attempt.